Aircraft heater with plenum chamber acoustical damping means



Nov. 15, 1949 H. J; DE N, MCCOLLUM 2,488,218

AIRCRAFT HEATER WITH PLENUM CHAMBER ACOUSTICAL DAMPING MEANS Original Filed Jan. l5, 1945 Patented Nov. 15, 1949 AIRCRAFT HEATER WITH PLENUM CHAM- BER ACOUSTICAL DAMPING MEANS Henry J, DeN, McCollum, deceased, late of Chicago, Ill., by Thelma McCollum, executrix, Chicago, Ill., assignol to SteWartWarner Corpora tion, Chicago, Ill., a corporation of Virginia Original application January 15, 1943, Serial No.

472,458. Divided and this application November 5, 1945, Serial No. .626,686

2 Claims.

The present invention relates to heaters, particularly of the aircraft type, and has for its principal objective to provide such a heater with a novel burner which cannot contaminate the Ventilating air by way of anycross connection between the burner and the Ventilating air sides of the heater.

Another object of the presentinvention is to provide a novel heater of the above type with a premix type burner which does not howl or run roughly even though it is sealed off from the Ventilating side of the heater.

Other objects and advantages will become apparent from the following description of a preferred embodiment of the invention which is illustrated in the accompanying drawing, in which Fig. 1 is a small fragmentary diagrammatic view of a heater installed in an airplane; and

Fig. 2 is an enlarged medial sectional View illustrating somewhat diagrammatically a heater embodying the features of the present invention.

The heater indicated generally by the numeral lil is located within and adjacent to the s lgin l2 of the airplane fuselage or wing. It is comprised of a generally cylindrical casing I4 which at its forward end is connected to an air duct I3 which terminates in a forwardly facing air ram or scoop l5 located outside the airplane, so that the dynamic effect of the airplane moving through the air causes a pressure rise within the scoop and Ventilating air duct se as to force Ventilating air through the heater to the space to be heated.

Within the case I4, the heater is provided with a heat exchanger i6 made up a bundle of thin walled tubes l 8 which are flanged over and sealed at their inlet ends to a circular tube sheet or header plate 23. Although not shown, a similar tube sheet at the downstream end of the heater supports and separates the tubes and closes oc the space therebetween so as to force ventilating air to flow longitudinally through the tubes from end to end of the heater. At the periphery, the tube sheet 2Q has a liange 22 secured to a cylindrical exhaust case 24 as by welding-the downstream tube sheet is similarly arranged-so that products o combustion entering the center of the heat exchanger from a burner 26 ow outwardly between the tubes toward the exhaust case 24 from which they exhaust to the atmosphere through an exhaust pipe 25 connected to the case near the downstream end vof the heater.

The burner 26 comprises a cup shaped chamber made of heat and corrosion resistant stainless steel or similar alloy, the open end of which projects through a iianged opening in the center of the tube sheet 20, the combustion chamber and tube sheet being secured ltogether by welding. The closed end of the combustion chamberI has a casting ,28 welded thereto which provides a pocket in communication with the interior of the burner. This pocket has a threaded opening at its upper end which receives an igniter 30 of the hot wire type, as explained in greater deetail in the copending application of Henry J. 'De N. McCollum, Serial No. 472,458, viilecl January 15, 1943, for Aircraft heating apparatus, of which this application is a division, and issued as Patent No, 2,427,221, dated September 9, 1947. In gene eral, the igniter comprises a coil of resistance wire which is heated suciently by an electric current to ignite a mixture of fuel and air. After combustion has been well started, it is usual prac:- tice to ole-energize the igniter so as to save elec.- tric power and to prolong the operating life of the igniter.

The end surface of the combustion chamber is also equipped with a thread-ed boss 32 which re..- ceives the complimentarily threaded head 34 of a re-igniter 3E. This re-igniter comprises a loose spiral coil of sheet Inconel or similar material welded to the head 34. During normal operation of the heater this coil becomes heated to in.- candescence and serves to, reignite the burner if for any reason combustion should termi-nate for a comparatively short period.

A combustion air pipe 38 extends into the cas,- ing I4 positioned upstream of the heater so as to receive air under pressure from the ventilating air duct i3. This air is passed by the com.- bustion air pipe 38 to a carburetor 40 of the balanced type having a connection, not shown, to a source of liquid fuel under pressure, so that combustion air passing through the carburetor has mixed therewith a proper l.quantity .of fuel to form a combustible mixture. This mixture passes from the carburetor to the burner through a mix-.- ture tube 42 which enters the burner tangentially and extends a portion of the way around the cir.-

cumference thereof, so that the combustible ture entering the burner will be preheated as an aid to rapid combustion. It desired, the com.- bustion air pipe 3S may be .connected to a separate ram instead vof being connected to the Ven.- tilating air duct as shown if it is desired to keep the Ventilating air side of the heater completely separate from the combustion air side.

In the work with aircraft heaters of the general type just described, it has been foundthat an acoustical phenomenon taires place, such that the burners rumble and sometimes howl with a comparatively high-pitched note. This matr make starting difficult, may cause the burner `to go out once it has been started and, in any event, makes combustion inelicient rand noisy. This phenomenon is likely to appear in almcst any new design and so far as known cannot be predicted in advance of testing. The phenomenon is particularly troublesome when as in the present case the burner is completely sealed from the combustion air inlet to the exhaust outlet, especially if the exhaust pipe and inlet connections are of substantial length. It has been found, however, that if the burner is opened to the air the howling and rumbling ceases, presumably because any pressure rise in the burner is quickly communicated to the outside through the openings, while any rarefaction similarly draws air into the burner. Thus, the sound waves do not reinforce themselves. Of course, the ordinary scheme of permitting combustion air to enter the burner immediately from the surrounding ventilating air as is used in most furnaces and small heaters serve this same purpose, so that as a rule this phenomenon is not troublesome excepting in sealed type burners.

In the heater of the present invention, however, this difficulty has been solved by providing the burner just downstream of the point where the major portion of combustion takes place with a plurality of holes 44 arranged circumferentially of the burner. These holes are enclosed with a plenum chamber 46 formed by an annular jacket which extends around the burner and is welded thereto at 48. This jacket is in turn provided with one comparatively large opening 50 which communicates with a tube 52 leading through the exterior skin I2 of the aircraft. Preferably the end of the tube 52 is scarfed off close to the skin I2 with the trailing edge of the tube projecting further than the leading edge into the air moving past the tube when the airplane is in motion so as to produce a pressure within the tube 52 slightly higher than in the combustion chamber. Although this pressure is not critical, it is an advantage to have what slight movement of air takes place through the tube 52 occur in an inwardly direction, so that heat from the burner 26 is not wasted as would be true if hot products of combustion were continuously flowing outwardly through the holes 44. The optimum ccnit has been found that a burner of this type will operate very quietly, even though it has considerable tendency to run rough with the holes d4 plugged. Further, it has been found that supplying the chamber 46 alone without the connection 52 to the outside atmosphere does not fully solve the problem, but adding the tube 52 does, even though the system may be so balanced that no air appears to flow through this tube in either direction.

Having described the invention, what is claimed as new and useful and desired to be secured by Letters Patent is:

1. In combination with a vehicle, a heating system for the vehicle, said heating system including means forming a cup-shaped combustion chamber, means supplying fuel to said combustion chamber adjacent the base thereof, means forming a heat exchanger connected to said combustion chamber at the outlet thereof, air ram means for supplying combustion air to said combustion chamber, means for supplying Ventilating air to said heat exchanger, a plurality of small ports in the wall of said combustion chamber forming means adjacent the outlet end of said combusion chamber, means forming a plenum chamber of relatively large volume enclosing said ports, and a conduit connecting said plenum chamber to the atmosphere, said conduit projecting into the air moving past the vehicle when the vehicle is in motion and having its trailing edge projecting further than its leading edge into the air moving past the vehicle whereby the movement of the vehicle will maintain a substantially static pressure in said conduit and plenum chamber substantially equal to that in said combustion chamber whereby acoustical disturbances in the heater will be absorbed in said plenum chamber.

2. In combination with an airplane, a heater comprising means forming a combustion chamber provided with a plurality of small ports in its wall,

dition would be reached if the pressure inside the -burner were exactly the same as that in the plenum chamber 46, so that no continuous flow in either direction through the openings ed would take place. It is not, however, at all necessary to reach this optimum condition in practice.

Preferably the plenum chamber @0 should have considerably more volume than would be necessary merely to jacket the holes 4. Or, to put it `another way, the chamber 46 should have low acoustical inertia, such that a momentary pressure rise within the combustion chamber will cause a momentary outflow through the holes fnl, vthereby raising the pressure in the chamber 45, and before this increase of pressure has had an opportunity to cause a directional movement in the pipe 52, the pressure impulse within the burner will have been reversed, so that the succeeding inflow through the holes 44 will cause a pressure drop in the chamber @5. It is apparent that the larger the chamber 46 the less the pressure will be raised therein by a momentary sound Wave impulse causing air to move outwardly through the holes 44. In general, in carrying out the invention, it is advisable to keep in mind that the quieting effect is brought about by the low resistance path which permits pressure impulses within the chamber to be quickly dissipated.

With the arrangement shown and described,

means including air ram means supplying a cornbustible mixture of fuel and air to said combustion chamber under pressure, means forming a heat exchanger adapted to receive the products of combustion from said combustion chamber, means forming a plenum chamber enclosing said ports, and a conduit connecting said plenum chamber to the atmosphere at the outer skin of the airplane, the outer end of said conduit being scarfed off close to the skin of the airplane with the trailing edge of said conduit projecting further than the leading edge into the air moving past said conduit when the airplane is in motion whereby the movement of the airplane will create a substantially static pressure in said conduit and said plenum chamber substantially equal to the pressure in said combustion chamber whereby acoustical disturbances in the heater will be absorbed in said plenum chamber.

THELMA MCCOLLUM, Eecutrz'cc of the Last Will and Testament of Henry J. De N. M cCollum, Deceased.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

